(1) Field of the Invention
The present invention relates to an optical amplifier used for an optical fiber transmission system and an optical signal processing system at various optical communication systems, in particular, suitably used for amplifying wavelength multiplexed light, and a controlling method for a Raman amplifier.
(2) Description of the Related Art
At present, as an optical amplifier for amplifying wavelength-multiplexed light, a rare earth element doped fiber amplifier is in general use. In particular, an erbium doped fiber amplifier (referred to as EDFA hereinafter) for C band whose amplifying band is 1530 to 1560 nm and an EDFA for L band whose amplifying band is 1570 to 1610 nm are well known for the fact that they exhibit excellent amplifying characteristics such as high pumping efficiency and low noise characteristic.
On the other hand, one of the problems to be solved for increasing the transmission capacity at a wavelength division multiplexing (WDM) optical transmission system is the expansion of band of the optical amplifier.
In order to expand the band, as shown in FIG. 11(a), two EDFAs mentioned above are connected in parallel by using an optical multiplexer and de-multiplexer (CL multiplexer, CL de-multiplexer) called a CL coupler.
In this regard, as shown in FIG. 11(b), an optical amplifier having an amplifying band of 1530 to 1610 nm has already been proposed and has been put to practical use as a commercial product.
However, there are three problems at this optical amplifier in which two EDFAs are connected in parallel by using the CL coupler. The first problem (1) is the deterioration of noise characteristic (noise factor) caused by inserting the CL coupler. The second problem (2) is the high cost caused by using two kinds of optical amplifiers being the EDFA for C band and the EDFA for L band. The third problem (3) is the high cost caused at the time when the number of operating channels is small, for example, at the initial installation stage.
To cope with the problems (1), (2) among those problems, expanding the amplifying band by one optical amplifier has been studied. It has been proposed that, for example, at the Raman amplifier in which an arbitrary amplifying band and an arbitrary amplifying bandwidth can be set, the bandwidth is expanded by multiplexing plural pumping wavelengths; the bandwidth is expanded by using a hybrid amplifier, combining an EDFA being well-established, in spite of being limited somewhat in the bandwidth, with a Raman amplifier, so as to constitute a hybrid amplifier (for example, in Japanese Patent Application Laid-Open No. HEI 11-84440); the bandwidth is expanded by using an EDF (erbium doped fiber) being multi-component glass.
However, expanding the bandwidth by using the multi-component glass poses several problems from the viewpoint of the reliability and the high non-linearity of the EDF. Therefore, in order to realize the expansion of the bandwidth, the Raman amplifier becomes a key amplifier.
In connection with the expansion of the bandwidth by using the Raman amplifier, the possibility of amplifying 200 nm bandwidth has been reported (refer to Toshiki Tanaka et al. “200-nm Bandwidth WDM Transmission around 1.55 μm using Distributed Raman Amplifier”, 28th European Conference on Optical Communication (ECOC) 2002, Post-Deadline Papers, Post-deadline session 4:PD4.6). However, the Raman amplifier has lower pumping efficiency than the rare earth element doped fiber amplifier. Therefore, at the Raman amplifier, larger pumping light power is required and the cost becomes high.